The invention relates to a method for the working of cavity walls of continuous casting molds.
For the production of continuous casting molds, in particular for the production of the geometry of the mold cavity in the case of tubular billet, bloom and profile formats, various production methods such as cold forming onto a mandrel or machining etc. are known.
The known production methods by means of cold or explosive forming onto a mandrel are expensive, because for each strand cross-section or each conicity shape a mandrel has to be manufactured, which particularly in the case of explosive forming has a short service life. A production by means of machining has its limitations, on the other hand, because the shapes of the mold cavities have become more and more complicated on continuous casting grounds. An additional difficulty is also caused with tubular molds, however, by the small ratio of the clear width to the length of the mold cavity, because the design of the working apparatus is thereby severely limited. In addition to molds with a straight mold cavity and with a casting cone uniform on all sides for a square or circular billet cross-section, molds with curved mold cavities for bow type continuous casting machines are mostly used today, which places an additional limitation on the dimensioning of the working apparatus.
In addition, use is made, in order to improve the strand quality and to increase the casting rate, of moulds with casting conicities varying in the longitudinal direction of the mold, for example with parabolic shape of the casting conicity. A further substantial improvement in the casting rate has been achieved by means of convex molds according to Konvex-Technologie, which is known from EP 0 498 296. In such molds the mold walls are provided on a part of the mold length with convex bulges which in the case of rectangular mold cavities taper into a flat wall, and in the case of circular mold cavities into a circular strand cross-section. In addition, mold cavities are known which exhibit smaller casting conicities in the corner areas than between the corner areas. Such mold cavities are incapable of being produced with known machining machine tools both because of the complex geometry on the one hand and because of poor accessibility in the tabular mold body on the other, and also because of the unfavourable ratio between mold length and clear mold cross-section.
A representative example of a machining machine tool suitable for the working of cavity walls is known for example from DE 1 577 330. DE 1 577 330 discloses a grinding machine for the working of the inner surfaces of steel work""s molds, i.e., molds for ingotting. The grinding machine incorporates a supporting arm whose longitudinal axis defines a middle position in a horizontal direction. The supporting arm is supported at one end on a trolley transportable in the direction of the middle position in such a way that the supporting arm is swivellable about a vertical axis and a horizontal axis aligned normal to the longitudinal axis of the supporting arm, and bears at its other end a grinding disc whose axis of rotation is arranged in horizontal direction oblique to the longitudinal axis of the supporting arm. Such an arrangement of the grinding disc permits the working of level inner surfaces, such as are conventional with steel work""s molds. Randomly bent inner surfaces, such as are conventional with continuous casting molds, and corner areas between bent inner surfaces may not be worked with the required accuracy with such an arrangement of the grinding disc.
The invention is based on the object of creating a method and an apparatus which are suitable for the inner working of mold tubes for billet, bloom and profile strands. In particular, mold cavities are to be producible with degrees of conicity varying along the mold, with parabolic conicity, with convex side walls, which taper onto a flat wall surface, or with special corner configurations with degrees of conicity of between 0 and 1%/m by machining and polishing work operations with a numerically controlled machine. In addition, a high mold cavity accuracy and surface quality are to be achieved and a cost-effective production method created on the basis of a controlled fabrication process which ensures automatic operation and a high machining rate with optimum chip removal.
The method according to the invention and the apparatus according to the invention make it possible for the first time, by means of a machining machine, to produce mold cavities for billet bloom and profile strands with a conicity varying along the mold, with parabolic conicity or with convexly bulging sidewalls with a numerically controlled machine in addition, it is possible to achieve by means of the method and the apparatus a high mold cavity accuracy and surface quality. Further advantages are a high degree of automation and a high machining rate with optimum chip removal out of the mold cavity. The sum of said advantages leads overall to a cost-effective production method for new molds or to a cost-effective re-working method for used molds and ones re-coated on the cavity side after use.
In Table 1: xe2x80x9cExamples of mold cavitiesxe2x80x9d at the end of the text it is intended to demonstrate the multiplicity of ways in which the configurations of mold cavities have developed and will also develop further in future. In addition to the cross-sections shown, tube molds for beam profiles such as xe2x80x9cDogbonexe2x80x9d are also to be described as difficult to work.
Molds may be clamped with their longitudinal axis substantially vertically onto a machine tool table and be worked with a vertical tool supporting arm. According to an embodiment it is advantageous if the mold is clamped onto a table with its longitudinal axis horizontally and the tool supporting arm is introduced into the mold cavity substantially horizontally. With such an arrangement the machine advantageously transports the arm with the aid of movement devices in a plane and the table along an axis normal to said plane.
The depth of penetration required for the tool supporting arm in the longitudinal direction of the tube may be reduced and in so doing the accuracy and surface quality of the worked surfaces be improved if, according to an embodiment, the table is after the working of about half the mold length, swivelled through 180xc2x0 about an axis which runs obliquely to the clamping plane of the table. By means of said additional process step the arm may be designed for a working depth of the mold cavity of 400 to 600 mm, i.e. for roughly half a mold length.
The relative movement between the tool and the mold cavity walls and the rotational movement of the arm about its longitudinal axis may be applied in many different combinations for the machining. According to a further embodiment it is possible in the case of square and circular mold cross-sections for all mold cavity shapes to be worked according to Table 1 if by means of the numerical control in a first step the arm is brought by a rotational movement about its own longitudinal axis into a working position at the mold cavity periphery and clamped and thereafter in a second step with the rotating tool a portion of the mold cavity surface is worked in a simultaneous movement in one, two or three spatial directions. Said sequence of steps may be continued until the whole of the mold cavity exhibits the desired geometry.
The service life of a mold tube may be prolonged quite substantially by repeated coating with a material and a subsequent machining and the mold costs per tonne of cast steel there by be reduced.
In order to improve the freedom of movement of the arm and the tool within the mold cavity, the arm is according to a further embodiment provided with a square cross-section with corner roundings and the tool is fixed at the end of the arm to a special tool holding disc so as to be exchangeable. In order to be able to dimension the cross-section of the arm as generously as possible for a particular mold cavity cross-section, in order on the one hand to increase the bending moment and on the other to prevent vibrations, it is additionally proposed to arrange the axis for the rotational movement of the tool at a distance from the longitudinal centre line of the arm. The distance of the axis of rotation of the tool is advantageously chosen as 10-25% of the diameter of a circle inscribable within the arm cross-section. A further advantageous optimization is obtained if the ratio of the rotational diameter of the tool to the rotational diameter of the arm lies in the range between 1:0.7 and 1:0.9.
In order to achieve a high polishing rate with large-area polishing tools, according to a further embodiment another arm with two axes of rotation arranged substantially obliquely to the longitudinal axis of the arm may be used for two disc-shaped polishing tools.
In order to transfer the drive for the machining and/or polishing tools from the machine tool table up to the axis of rotation of the tool, it is proposed, according to an embodiment, that an axial drive shaft with tooth gears be provided in the arm, in order to obtain a torsion-proof force transmission. A belt drive for the tools is conceivable as an alternative.